Combined cycle power plants, which are well known in the art, generate electricity by using both gas turbines and a steam turbine. The gas turbines operate in a conventional manner, but what would otherwise be the wasted, hot exhaust gas from the gas turbines is used by at least one and usually more heat recovery steam generators (hereinafter "HRSG") to produce steam to run the steam turbine. As a result, a generator driven by the steam turbine produces additional electricity without the expenditure of any additional fossil fuel, unless some small amount is used for the supplementary heating of the exhaust gas prior to its reaching the HRSGs. Such plants and the general control systems therefor are described in Baker et al. U.S. Pat. No. 3,879,616 and in Wagner et al. U.S. patent application Ser. No. 187,153 now U.S. Pat. No. 4,329,592, filed Sept. 15, 1980, both assigned to the same assignee as this application and both incorporated herein by reference.
Maximum plant output is achieved under steam and gas operation which normally occurs during the day. For the usual low demand periods, e.g., nighttime, the steam turbine system may be shut down, and the plant run in a simple cycle mode with only the gas turbines operating. Thus, the daily cyclic operation of the steam turbine system may involve starting some or all of the HRSGs and the steam turbine from either a cold state or some intermediate state of readiness. As a result, this daily start-up necessarily subjects the HRSGs, the steam turbine and related equipment to large thermal gradients which induce substantial mechanical stresses. As these thermal stresses can severely reduce the useful life of the components, the prior art control systems for the combined cycle plants reduce the thermal gradients as much as possible by using a very gradual start-up for the steam turbine system. It is, however, much more efficient if the start-up is rapid, thereby putting the steam turbine on-line as quickly as possible.
In addition to the thermal stress problem, start-up time is often lengthened by certain transient conditions, one of the most critical of which involves steam drum water level control. Each HRSG has a steam drum which is connected to an evaporator, which is heated by the exhaust gas. Under normal operating conditions, the evaporator continuously feeds both water and steam to the drum. At start-up, however, the evaporator only contains water, and initial boiling may occur at different places in the evaporator, forcing large slugs of water into the drum. The sudeen increase in drum water, or drum level surge, is undesirable as it may force water into a steam super-heater and the steam lines. Thus, in the prior art, the boiling is done very slowly, and the water level in the drum is maintained by dumping water through motor-operated drain valves. The latter is an additional drawback in that in correcting drum level surge problems large quantities of the expensive, chemically-treated water are lost.
Finally, while the HRSG is being brought on-line, it is desirable to generate steam at a rate which is compatible with the load rate limitations of the steam turbine. The prior art controls, however, do not do this.
Accordingly, one object of the present invention is to provide an improved control system for the steam turbine system of a combined cycle plant, which control system will permit rapid start-up of the plant while automatically limiting thermal stresses and compensating for transient start-up conditions.
Another object of the present invention is to provide such a control system which will automatically vary the rate of steam generation to produce the desired steam turbine loading rate.
Other objects, advantages, and features of the present invention will become apparent from the following description of the preferred embodiment taken together with the drawings and claims.